In a typical cellular system, also referred to as a wireless communications network, wireless terminals communicate via a Radio Access Network (RAN) to one or more core networks. The wireless terminals may be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called Evolved Node B (eNB), NodeB or B node and which in this document also is referred to as a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. A base station communicates over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units. The Third Generation Partnership Project (3GPP) has undertaken to further evolve the UTRAN and GSM based radio access network technologies. In 3GPP this work regarding the 3rd Generation (3G) Long Term Evolution (LTE) system is ongoing.
Pre-coding is a technique which supports multi-layer transmission in multi-antenna wireless communications. In short, a transmitter sends coded information to a receiver in order to the pre-knowledge of the channel. In single-layer beamforming, the same signal is emitted from each of the transmit antennas with appropriate weighting such that the signal power is maximized at the receiver output. When the receiver has multiple antennas, single-layer beamforming cannot simultaneously maximize the signal level at all of the receive antenna. Thus, in order to maximize the throughput in multiple receive antenna systems, multi-layer beamforming is required.
Single-layer transmission refers to transmission of the same signal is emitted from each of a plurality of antennas, and multi-layer transmission refers to transmission of multiple data streams.
In point-to-point systems, pre-coding means that multiple data streams are emitted from the transmit antennas with independent and appropriate weightings such that the link throughput is maximized at the receiver output. In multi-user Multiple Input Multiple Output (MIMO), the data streams are intended for different users and some measure of the total throughput, e.g., the total cell performance is maximized. In point-to-point systems, some of the benefits of pre-coding may be realized without requiring channel state information at the transmitter, while such information is essential to handle the co-user interference in multi-user systems.
When code book based pre-coding is used a selected set of pre-coders are defined in a table, the code book. When code book based pre-coding is used a selected set of pre-coders are defined in a table, the code book. A pre-coding matrix indicator (PMI) refers to a referring index into that code book. Pre-coding may also be selected unlimited non discrete or in a non-discrete manner, i.e. called non-code book based pre-coding. PMI or other type of indicator which enables selection of a pre-coding vector may be applied to wireless transmissions in a communication network as a feedback metric used for MIMO pre-coding. The pre-coding process is used to optimize the quality of the signal at the receiver. The following PMI reporting is described for LTE, as an example, but similar reporting procedure applies for e.g. Worldwide Interoperability for Microwave Access (WiMAX) as well.
In the LTE closed-loop spatial multiplexing mode, i.e. with a code book based pre-coding, a base station selects a pre-coder matrix from a predefined codebook with the help of a user equipment's (UE) suggestion in the shape of one or more pre-coding matrix indicators signaled as part of the channel knowledge or information.
LTE is an OFDM (Orthogonal Frequency Division Multiplexing) access. The bandwidth is divided into a number of 15 kHz sub-carriers, orthogonal frequency. 12 such sub-carriers are further grouped into a sub-band, i.e. resource block, of 180 kHz. These sub-bands are the resource block for scheduling and channel state reporting, such as PMI.
The user equipment may be configured to send PMI reports either periodical, e.g. on Physical Uplink Control Channel (PUCCH), or scheduled, e.g. on Physical Uplink Shared Channel (PUSCH). The PMI for the last reporting period may to a various degree of accuracy indicate forthcoming channel quality. However, e.g. user equipment position, user equipment speed and changes in the local propagation environment will alter the fast fading. This makes it clear that the estimated PMI has better short-term than long term accuracy, and that a long delay between measurement and PMI usage may reduce potential gains. Typical shortest feasible delay between measurement and application of PMI is e.g. 10 to 15 ms. For example, the reporting delay may be 6 ms and the reporting interval may be in the order of 5 to 40 ms.
The larger the pre-coding frequency granularity, the larger the gain is but at the cost of larger the reporting overhead. It is desired to increase the reporting period to save uplink radio resources.
The user equipments bases PMI reports on measurements on downlink reference signals. PMI reports are transmitted on uplink control channels to the eNodeB. PMI reporting is comprised in the Channel State Information (CSI) report. Depending on CSI report configuration, a CSI report may comprise of a PMI rank indicator (RI), and CQI (Channel Quality Indicator). The PMI may be reported wideband or frequency selective.
As a user equipment moves, the radio channel will be affected by the altering fast fading. For a given frequency, i.e. sub-band, the channel will fade in the time domain and the fading speed is primarily dependent on the user equipment speed. If considering a specific point in time, one will have more or less similar occurrence, but in the frequency domain instead.
Due to the delays involved in the CSI reporting procedure, the sub-band PMI to be used for pre-coding will be more or less outdated. Having a too outdated PMI will be similar to having a random selection procedure or to have transmit diversity, that is open-loop spatial multiplexing in LTE. This is described in FIG. 1, showing user equipment speed impact on closed loop pre-coding efficiency. The x-axis of FIG. 1 shows mobile speed in km/h. The y-axis of FIG. 1 shows normalized system tp and normalized cell-edge user tp. The line marked with triangles refers to Single Input, Multiple Output (SIMO), the line marked with squares refers to Alamouti codes, the line marked with circles refers to a closed-loop multiplexing and the line marked with diamonds refers to open-loop multiplexing. Alamouti codes is a MIMO transmit diversity scheme for two transmit antennas that does not require transmit channel knowledge.
It is known in that prediction can mitigate the PMI deterioration due to delays. In the patent document US 2007/0206626 it is describes a method of predicting future PMIs at the receiver based on a history of channel estimates, where the receiver additionally feeds back the predicted PMI values to the transmitter. One drawback of such a method is the added complexity in the receiver, which may create an undesirable increase of cost and power consumption. Another drawback is that the prediction is restricted to PMIs which can be fed back to the transmission using efficient signaling, i.e. PMIs belonging to a pre-determined codebook.